Sulfur-vulcanizable rubber compounded stocks based on conjugated diene polymers have employed carbon black in one or other of its various forms as the reinforcing filler of choice to provide the usually desirable jet-black color, as a filler (diluent), and to add desirable properties of life, abrasion resistance, and the like, particularly for making tires. The carbon blacks have been readily available and relatively cheap.
However, in recent years, the costs of the available base oils/natural gas for producing carbon blacks have been increasing radically. There also has been a trend toward lighter-colored vulcanizable stocks for some purposes.
As a consequence, the use of siliceous fillers such as silica in sulfur-vulcanizable rubber compounds has been receiving increasing attention. Not too long ago siliceous fillers were considered "high cost", and not likely ever to replace carbon black more than fractionally, particularly in view of some difficulties experienced in obtaining satisfactory results in sulfur vulcanizable stocks. However, the costs of siliceous fillers are dropping with increasing usages and improved technology.
Use of siliceous fillers, however, requires a coupling agent to adequately bond the siliceous filler to the rubber to provide sulfur-vulcanizable rubber vulcanizates with desirable tensile strength, modulus, and hardness. The silane coupling agents, however, reduce the time-to-scorch of the rubber compounds during curing, making processing such as extrusion and injection molding of the rubber compounds very difficult. Efforts have been made to add still another component to the compounded stocks to overcome the disadvantages. These have not been too successful, and every added component means added complexities in compounding since each new component tends to affect the balance of the other components.